(3.) Ascertain what power the stone has to resist abrasion. This is done in France by putting the broken metal into a revolving cylinder and then carefully noting by weight what the cubes lose by contact with each other. Another plan may be adopted by pressing the stone against a grindstone with a uniform pressure, and noting the loss caused by such contact. (4.) The power to resist compression may be easily ascertained by placing small cubes in an hydraulic press and noting under what pressures each cube will crush. (5.) The effect of weather is not easily ascertained artificially, although it is suggested that a good test may be made by soaking the stone in a saturated solution of sulphate of soda; and then on exposure to the air, if soft, it is said the stone will disintegrate as if under the action of thaw succeeding frost.[27] The specific gravity of a stone is no criterion whatever as to its fitness. Clay-slate has a higher specific gravity than a tough flint, and yet the former is almost useless as a road metal; the latter, on the contrary, often making excellent roadways. The qualities necessary for a really good road metal are hardness, toughness, not easily decomposed or affected by the weather, and at the same time the stone when broken ought to have some power of cohesion without the necessity of much binding material. The question of cost I put aside at once, as it is well known that the best road metal is always the cheapest where there is much or heavy traffic. Local circumstances must to a great extent determine what stone to use upon a roadway, but the following list may be of use:-- _Syenite._--This is a granite in which hornblende takes the place of mica, and is an excellent road material; the darker the colour the more durable it is found to be. _Granite._--This should have more felspar than quartz, and have as little mica as possible; the closer the grain the better. Coarse-grained granites soon decompose. _Trappean Rocks._--Some of these are excellent for road metal. Basalts of dark colour and close grain should be selected. Greenstones with similar characteristics are good; as is also Whinstone. _Gneiss._--Is inferior to granite; it has mica in layers and is not a good road metal. _Clay Slates._--These are useless, as they crumble on exposure or degenerate into mud. _Limestone._--The Metamorphic, Silurian, and Carboniferous limestones may be used if crystalline in appearance, but the Lias and Oolitic are of little use.[28] _Sandstones._--Some of these, if cherty or containing a large percentage of iron, may be used; but as a rule they are quite unfitted for use as a road metal. _Flints._--These, if tough, make excellent roadways; but unfortunately they are sometimes too brittle for heavy traffic. Surface-picked flints are better than those from a quarry.[29]